fix compile error

fix compile error:

    collection.scala:159: too many arguments for constructor RenameCollection: (name: String, target: String, dropTarget: Boolean)reactivemongo.core.commands.RenameCollection
    [error] def rename(to: String, dropExisting: Boolean = false)(implicit ec: ExecutionContext): Future[Boolean] = db.command(new RenameCollection(db.name + "." + name, db.name + "." + to, to, dropExisting))